Modern high data-rate modems perform a significant amount of signal processing in order to maximize utilization of available bandwidth. The signal processing may be performed on analog signals or digital signals or more typically, on both analog and digital signals. Filtering and channel equalization are the two most commonly used signal processing operations performed on data in every modem.
Filtering is used to separate the desired signal from other signals, such as noise and other forms of interference may be performed via low-pass filters, high-pass filters or band-pass filters, depending on the location of the desired signal with respect to the noise and interference. Analog filters are made from actual capacitors and resistors, for example. Digital filters, on the other hand, are software filters written like programs and specified by their filter coefficients. Digital filters execute like programs on a dedicated digital signal processor (DSP), a general purpose DSP, or a general purpose microprocessor or may be implemented as firmware on a custom designed piece of hardware.
Channel equalization is performed by channel equalizers and can be done in either the analog or digital domains. Channel equalizers attempt to flatten out a channel's frequency response. Because a channel's frequency response tends to rapidly attenuate as the frequency increases, channel equalizers compensate for the frequency response attenuation by imparting an increasing amount of gain corresponding to the increasing frequency. Analog channel equalizers are created from amplifiers, capacitors and resistors, for example. Digital channel equalizers are created in software and like digital filters may be programs executing on a dedicated DSP, a general purpose DSP, or a general purpose microprocessor or be implemented in firmware on a custom designed piece of hardware.
Each filter and channel equalizer has a distinct set of parameters which specifies their behavior. For example, a filter may be specified by its type (low-pass, band-pass, or high-pass), order (how rapid the transition is from the pass-band to the stop-band) and corner frequency (the frequency where the pass-band begins to transition to the stop-band). A channel equalizer may be specified by the slope of its frequency response. The channel equalizer's frequency response is typically the inverse of the frequency response of the channel that the channel equalizer is attempting to equalize.
Traditionally, the parameters for each filter and channel equalizer in a modem are set independently of one another. Normally, the modem, either through information provided to it or via some measurements it made on its own, will set the parameters based on channel characteristics. However, by setting the various parameters independently, no regard is given to how the filters and channel equalizers interact. For example, a receive filter set to operate as a high-pass filter to provide some echo rejection also affects the overall channel response which in turn affects the performance of the channel equalizer. A hardware equalizer increases the dynamic range for a high frequency signal and also provides some echo rejection, but it also affects the overall channel response and hence the performance of an adaptive channel equalizer.
Techniques that set parameters for the filters and channel equalizers without regard for the impact of the filters and channel equalizers on the channel characteristics may lead to a lower overall performance level. A need has therefore arisen for a technique that sets the parameters of the filters and channel equalizers with consideration of the impact on the channel characteristics due to these individual components.